The field of the invention is that of machines for dispensing a paste onto a workpiece.
In metal stencil mask printing processes, paste is extruded through a stencil mask by means of an extrusion nozzle in contact with the mask. This extrusion is accomplished through the use of a pneumatic piston applying pressure to a sliding plunger within the paste reservoir chamber.
Typically, the mask has the same width as the workpiece, such as a printed circuit board, a ceramic chip carrier for holding integrated circuits, etc. The nozzle travels across the mask, dispensing a thick paste that passes through the mask according to the pattern on the mask and is deposited on the workpiece.
As the nozzle traverses the surface of the mask, the plunger moves down and paste is dispensed through the nozzle. The paste is extruded through the metal mask onto the ceramic greensheet, in the case of chip carriers, filling through vias and printing metallized lines.
In this process, there are a number of conditions that can lead to screening problems:
a) Entrapped air bubbles in the paste causing random screening defects (line voids);
b) Nozzle and screener variability causing paste dispense rate variation that can result in yield losses and process control difficulties;
c) Interior reservoir wall defects, piston defects, and air cylinder misalignment/defects that can create binding or “hiccups” (stiction/friction) in plunger displacement; and
d) Screener or nozzle malfunctions can lead to gross paste overusage.
Presently, there is no good method for detecting any of the above problem conditions:
a) Entrapped air bubbles go largely unnoticed during the screening process. The entrapped bubbles can lead to small screening defects that are not detected until pattern inspection;
b) Paste dispense rate variation is currently undetectable in screening. This problem increases the variability of the manufacturing process, lowering overall product yields;
c) Stiction/friction and binding problems are only detectable through manual inspection of reservoir walls, piston, and air cylinder and their linkages and alignment; and
d) Gross paste overusage is usually not recognized until an entire paste reservoir has been wasted on just a handful of sheets (normal paste usage results in 50 60 sheets per reservoir).
Various mechanisms are known in the field of dispensing liquids. For example, U.S. Pat. No. 4,662,540 illustrates a pressure sensor that sends a signal representative of the pressure in the fluid reservoir to electronics that sound alarms when various undesirable situations occur. As one instance, the electronics sounds an alarm when the pressure exceeds a desired higher or lower limit.
Bubbles are detected by a rate of change in pressure that exceeds a limit.
The foregoing approach implicitly assumes that the cross section of the dispensing system is constant.
In the field of paste dispensing, through a mask, the cross section of the pattern is a variable and therefore the “impedance” of the system will also vary.
In addition, the paste reservoirs are not constant in time, but change as different batches are made up from clean component parts and put into the line.
Furthermore, the use of pressure sensors is not practical in this instance because of the difficulty in properly mounting them in communication with the paste and the difficulty in cleaning them and protecting them from malfunction due to dried paste.
Thus, the field of paste dispensing has not had available a satisfactory monitoring system.